1. Field of the Invention
The present invention relates to a rangefinder apparatus, more specifically, to a rangefinder apparatus comprising an AF (Auto Focus) sensor of passive type, and a camera equipped with the rangefinder apparatus.
2. Related Background Art
Conventional rangefinder apparatuses determine the reliability of a correlation local minimum value, shift amount of which is used for calculating the distance to the object, in order to avoid erroneous rangefinding.
For example, a rangefinder apparatus disclosed in Japanese Patent No. 2707569 determines the reliability of a local minimum value detected first while changing the shift position of the window areas by checking the difference between gradients, one being the gradient between the correlation local minimum value and a correlation value of the previous shift position, the other being the gradient between the correlation local minimum value and a correlation value of the following shift position. Also, in this rangefinder apparatus, a new correlation local minimum value obtained by interpolation of the respective correlation values of the shift positions before and after the correlation local minimum value is divided by the greater gradient among the above-mentioned gradients, and thus obtained normalized interpolated correlation local minimum value is compared with a reference value so as to determine the reliability of the correlation local minimum value.
When the above-mentioned two determinations are favorable, the conventional rangefinder apparatus adopts the correlation local minimum value detected first for calculating the distance to the object. Therefore, when a repeating pattern such as the one shown in FIG. 25A is subjected to rangefinding, for example, it chooses the result of a correlation shift region SI shown in FIG. 25B initially subjected to the reliability determination. As shown in FIG. 26, when the difference in correlation value gradients around the correlation local minimum value detected first (the correlation local minimum value on the right side in FIG. 26) and a new interpolated correlation local minimum value obtained by interpolation are favorable, the correlation local minimum value detected first (the correlation local minimum value on the right side in FIG. 26) is adopted for calculating the distance to the object without carrying out shifting to the area yielding the smallest correlation local minimum value (the correlation local minimum value on the left side in FIG. 26), whereby causing rangefinding error.
Thus, when a plurality of correlation local minimum values exist in correlation values, a correlation local minimum value which really is not the smallest correlation local minimum value may be adopted as the smallest correlation local minimum value. When the distance to the object is calculated based on the shift amount of thus erroneously adopted correlation local minimum value, the erroneous rangefinding occurs.
In view of the problem mentioned above, it is an object of the present invention to provide a rangefinder apparatus which can suppress erroneous rangefinding, and a camera equipped with such rangefinder apparatus.
The present invention provides a rangefinder apparatus comprising AF data generating means for forming an image of light from an object to be subjected to rangefinding onto a pair of line sensors each including a plurality of light-receiving elements, and generating AF data for computing a correlation value according to signals obtained from the light-receiving elements, AF data acquiring means for acquiring the AF data from a pair of employed sensor areas used for rangefinding in the pair of line sensors; correlation value computing means for determining a pair of window areas for selecting the AF data to be used for computing the correlation value within the pair of employed sensor areas, and successively computing correlation values while shifting the pair of window areas; interpolated correlation extreme value computing means for detecting a correlation extreme value(s) among the correlation values computed by the correlation value computing means, and interpolating thus detected correlation extreme value(s) so as to compute an interpolated correlation extreme value(s); highest correlation value detecting means for detecting as a highest correlation value the interpolated correlation extreme value exhibiting the highest correlation among the interpolated correlation extreme value(s); shift amount computing means for computing a shift amount of the window areas yielding the highest correlation value; and object distance calculating means for calculating a distance to the object according to the shift amount computed by the shift amount computing means.
This rangefinder apparatus detects a correlation extreme value(s), and interpolates it (them) so as to compute an interpolated correlation extreme value(s). When a plurality of correlation extreme values exist, interpolated correlation extreme values are more reliable for detecting the real smallest correlation extreme value as the highest correlation value. The smallest interpolated correlation extreme value is detected as the highest correlation value, and the shift amount of the highest correlation value is computed. Therefore, as compared with the conventional techniques detecting the highest correlation value from correlation extreme values before interpolation, there is lower possibility to erroneously calculate the distance to the object.
The xe2x80x9cAF dataxe2x80x9d in the present invention include the light intensity data obtained from respective light-receiving elements and the data obtained by processing the light intensity data. Such data processing includes contrast extraction processing that is described in detail hereinafter.
Preferably, the rangefinder apparatus in accordance with the present invention is the one further comprising oscillation degree calculating means for calculating a value indicative of an oscillation degree of the AF data; the interpolated correlation extreme value computing means computing the interpolated correlation is extreme value(s) on the condition that the value indicative of the oscillation degree calculated by the oscillation degree calculating means is greater than a predetermined reference value; and the highest correlation value detecting means detecting the highest correlation value from the correlation extreme value(s) that is not interpolated when the value indicative of the oscillation degree calculated by the oscillation degree calculating means is at or smaller than a predetermined reference value.
Preferably, the rangefinder apparatus in accordance with the present invention is the one further comprising comparing means for detecting a first correlation extreme value exhibiting the highest correlation and a second correlation extreme value exhibiting the second highest correlation in the correlation extreme values that are not interpolated, and judging whether a difference between the first correlation extreme value and the second correlation extreme value normalized by the first correlation extreme value is adequately large by comparing a ratio of the second correlation extreme value to the first correlation extreme value with a predetermined reference value; the interpolated correlation extreme value computing means computing the interpolated correlation extreme value(s) on the condition that the comparing means judges that the difference is not adequately large; and the highest correlation value detecting means detecting the highest correlation value from the correlation extreme value(s) that is not interpolated when the comparing means judges that the difference is adequately large.
By omitting the interpolation computing in case the reliability of the correlation extreme values is verified by the oscillation degree calculating means or the comparing means, overall computation load can be reduced.
Preferably, the rangefinder apparatus in accordance with the present invention is the one further comprising first determining means for determining the validity of the interpolated correlation extreme value(s) computed by the interpolated correlation extreme value computing means; the highest correlation value detecting means detecting the highest correlation value from the correlation extreme value(s) that is not interpolated when the first determining means determines that the interpolated correlation extreme value(s) is invalid.
Preferably, the rangefinder apparatus in accordance with the present invention is the one further comprising second determining means for determining the capability to calculate the distance to the object by comparing the difference between a first interpolated correlation extreme value exhibiting the highest correlation and a second interpolated correlation extreme value exhibiting the second highest correlation in the interpolated correlation extreme values computed by the interpolated correlation extreme value computing means with a predetermined reference value.
By withholding the adoption of unreliable interpolated correlation extreme values by means of the first determining means or the second determining means, the rangefinding becomes more accurate.
Preferably, the rangefinder apparatus in accordance with the present invention is the one wherein the second determining means changes the predetermined reference value according to the first interpolated correlation extreme value. It becomes possible to determine the capability to calculate the distance to the object further in detail by this configuration.
Preferably, the rangefinder apparatus in accordance with the present invention is the one wherein the shift amount of the window areas yielding the highest correlation value based on the interpolated correlation extreme value(s) is computed using an arithmetic expression that is used in computing the interpolated correlation extreme value(s). Accurate shift amount can be obtained by this configuration.
The present invention provides a camera comprising any of the rangefinder apparatus mentioned above. The camera is not only still cameras such as film cameras and digital still cameras, but also video cameras.